Interpretive Summary: Desert rodents have long been assumed to limit grass recruitment once shrubs have invaded grasslands, but there have been few tests of this assumption. We tested the hypothesis that herbivore-related mortality of seedlings of the dominant perennial grass Bouteloua eriopoda would be highest in shrub-dominated areas. We tested the hypothesis in two Chihuahuan Desert sites featuring similar shrub encroachment patterns but different shrub species, grass cover, and different abundances of small mammals. We found that seedlings were killed by mammals in greater numbers in shrubland than in grassland or mixed areas at the site with large herbivore numbers. At the site with low herbivore numbers, most seedlings were killed in mixed grass/shrub types. The abundance patterns of herbivores did not parallel patterns of seedling herbivory, however. Seedling herbivory is an important process and is related to vegetation composition, but the mechanisms underlying the relationship are not clear and may be related small mammal foraging behaviors. Small mammals have large impacts on grass recruitment.

Technical Abstract:
Does shrub invasion at ecotones indirectly limit grass establishment by increasing mammalian seedling herbivory, Chihuahuan Desert, New Mexico? We tested the hypothesis that herbivore-related mortality of seedlings of the dominant perennial grass Bouteloua eriopoda would be highest in shrub-dominated portions of grassland-shrubland ecotones. We tested the hypothesis in two Chihuahuan Desert sites featuring similar shrub encroachment patterns but different shrub species, grass cover, and different abundances of small mammals. Within each site we transplanted B. eriopoda seedlings to grass-dominated, middle, and shrub-dominated positions of replicate ecotones during the time of year (mid-summer) when they would naturally appear and monitored seedling fates. We estimated population size/activity of putative small mammal herbivores. Seedlings were killed by mammals in greater number in shrubland than in grassland or middle ecotone positions at the site withlarge herbivore numbers. At the site with low herbivore numbers, most seedlings were killed in middle ecotone positions. The abundance patterns of herbivores did not parallel patterns of seedling herbivory across the ecotones or between sites. Seedling herbivory is an improtant process and is related to vegetation composition, but the mechanisms underlying the relationship are not clear. We speculate that variation in small mammal foraging behvior may contribute to seedling herbivory patterns. Restoration strategies in the Chihuahuan Desert need to account for the abundance and/or behavior of native herbivores.